1. Field of the Invention
This invention relates to a process for reducing the tendency of a gas stream containing elemental phosphorus, to corrode metallic surfaces in a combustion zone upon combustion of the gas stream; particularly a gas stream containing substantially carbon monoxide, and particularly that gas stream obtained from the electric furnace production of elemental phosphorus.
2. Prior Art
In the electric furnace process for preparing phosphorus, phosphate-bearing ore, rock or earth is charged to an electric furnace with silica and carbon, usually in the form of coke or coal. The electric furnace is heated to a sufficient temperature to melt the charge. The phosphorus generated is removed as a vapor with carbon monoxide gas. The silica is used as a flux for the calcium present in the phosphate rock. An approximate equation for the over-all reaction is given by: ##STR1## The gas, as it leaves the furnace entrains minute particles of carbon, phosphate rock and other materials present in the furnace. This phosphorus containing gas stream is then generally passed to a hot gas precipitator which removes a large portion of the entrained solids. The gas stream is then conveyed to a hood type condenser where the elemental phosphorus (P.sub.4) is condensed, and then the gas stream exhausted through a spray tower.
In some installations, the entrained solids and dust are removed by, for example, Cottrell precipitators which are operated above the dew point of the elemental phosphorus, while in other installations the entrained solids and dust are removed by impingement. The phosphorus is often condensed in spray towers equipped with water sprayers kept at 45.degree. C. and then run into a sump where small amounts of impurities called mud, are precipitated. The liquid phosphorus is finely pumped into storage tanks where the last pieces of mud fall to the bottom.
Typically, the gas stream emanating from such a process consists of 93% carbon monoxide, the remainder being primarily elemental phosphorus (P.sub.4) with some silicon tetrafluoride and dust. The carbon monoxide may be recycled and burned in kilns used for sintering or agglomerating the phosphate bearing ore, or in other process steps.
A problem occurs when, for example, one of the kilns is not operating or the electric furnace used to heat the charge is operating at high capacity. At such times excess carbon monoxide is available for use. Currently, in most plants, this excess gas is flared to the atmosphere. A problem in utilizing this excess gas, for example, to generate steam and/or electric power in a boiler for use in the plant is that the phosphorus burns to acid, for example, meta-phosphoric acid (HPO.sub.3), in the presence of water, for example: EQU P.sub.4 +2H.sub.2 O+5O.sub.2 .fwdarw.2P.sub.2 O.sub.5 +2H.sub.2 O.fwdarw.4HPO.sub.3
The acid, e.g. meta-phosphoric acid (HPO.sub.3),
produced is very corrosive to metallic surfaces. The acid has a high condensation temperature, or dew point. This is the primary reason that such carbon monoxide gas steams are generally not utilized in boilers to generate steam and/or electric power for the elemental phosphorus content of the carbon monoxide gas stream is such that when oxidized to acid; the acid condenses on the surface of the boilers, forming a glaze, which is highly corrosive.
To date, there is, to applicant's knowledge, no known economical, effective and practical process for reducing the elemental phosphorus content of the carbon monoxide gas stream to levels which will permit the combustion of the carbon monoxide gas stream in a combustion zone suitable for generating steam and/or electric power, said combustion zone having metallic surfaces.